[Tiered Caching Stats] [WIP] Cache Stats API

[peteralfonsi]

Description

Adds a new cache stats API at /_nodes/stats/caches. For now, the supported cache is request_cache. Supports aggregation by levels (for request cache, this is indices, shards, or tiers.)

Related Issues

Followup to opensearch-project#12531

Check List

• New functionality includes testing.
  • All tests pass
• New functionality has been documented.
  • New functionality has javadoc added
• Failing checks are inspected and point to the corresponding known issue(s) (See: Troubleshooting Failing Builds)
• Commits are signed per the DCO using --signoff
• Commit changes are listed out in CHANGELOG.md file (See: Changelog)
• Public documentation issue/PR created

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